The thermal conductivity of diamond is not fixed; even for the same type of diamond, its thermal conductivity falls within a certain range of variation. Overall, the thermal conductivity of different types of diamonds is primarily influenced by the following core factors:

Integrity of the internal structure (structural defects): Diamond's thermal conductivity is extremely sensitive to defects in its crystal structure. The more complete the internal crystal structure, the higher the thermal conductivity generally is. For example, one of the core characteristics of Type IIa diamond, which has excellent thermal conductivity, is its relatively complete crystal structure. Conversely, if Type Ib diamond is produced using insufficiently advanced processes, its thermal performance will not be ideal due to a large number of internal structural defects.

Types and contents of impurities: Chemical impurities mixed inside the diamond directly become obstacles to heat propagation. Taking nitrogen as an example, the reason Type IIa diamond has extremely high thermal conductivity is precisely because of its very low nitrogen content. Additionally, if a large amount of metallic inclusions is produced during the artificial synthesis process, it will also significantly weaken the diamond's heat conduction capability.

Environmental temperature conditions: External temperature is a dynamic factor that determines thermal conductivity, and the same diamond exhibits vastly different thermal conductivity performances at different temperatures. For instance, the thermal conductivity of Type IIa diamond is 2000–2200 W/(m·K) at 273K (about 0°C), whereas at a low temperature of 80K, its thermal conductivity can soar to 15000 W/(m·K).

Secondary defects introduced by the sintering process (for polycrystalline sintered bodies): When single-crystal diamonds are processed into polycrystalline diamond sintered bodies, process control during sintering directly affects the final thermal conductivity. Grain boundary impurities, pores, other defects, and additives generated inside the sintered body will further obstruct heat propagation, leading to a decline in the material's overall thermal performance.

In conclusion, to maximize the potential of using diamond as a heat spreading material, the key lies in reducing crystal structural defects and lowering impurity contents through advanced manufacturing processes, while simultaneously avoiding the introduction of new thermal propagation barriers, such as pores and grain boundary impurities, during the sintering process.